For architectural flat glass, such as is made by the "float" process, two of the more prominent techniques for creating solar management coatings on these glasses are the pyrolytic process and the magnetron sputter-coating process. Drawbacks heretofore experienced in the sputter-coating process have been that the coatings can often be easily rubbed off (i.e. lack "durability") and that the polysealant used in forming multi-paned architectural windows often attacks the coating. This, in turn, breaks down the seal between the panes, allowing detrimental condensation to accumulate between them. On the other hand, sputter coatings have had the historic advantage of being able to achieve low emissivity values and high visible light transmittance properties, as compared to most pyrolytic coatings. These latter two properties are perhaps among the most important to achieve in certain architectural glasses.
The terms "emissivity" and "transmittance" are well understood in the art and are used herein according to their well known meaning. Thus, for example, the term "transmittance" herein means solar transmittance, which is made up of visible light transmittance, infrared energy transmittance, and ultraviolet light transmittance. Total solar energy transmittance is then usually characterized as a weighted average of these other values. With respect to these transmittances, visible transmittance, as reported herein, is characterized by the standard Illuminant C technique at 380-720 nm; infrared is 800-2100 nm; ultraviolet is 300-400 nm; and total solar is 300-2100 nm. For purposes of emissivity, however, a particular infrared range (i.e. 2,500-40,000 nm) is employed, as discussed below.
Visible transmittance can be measured using known, conventional techniques. For example, by using a spectrophotometer, such as a Beckman 5240 (Beckman Sci. Inst. Corp.), a spectral curve of transmission at each wavelength is obtained. Visible transmission is then calculated using ASTM E-308 "Method for Computing the Colors of Objects by Using the CIE System" (Annual Book of ASTM Standards, Vol. 14.02). A lesser number of wavelength points may be employed than prescribed, if desired. Another technique for measuring visible transmittance is to employ a spectrometer such as a commercially available Spectragard spectrophotometer manufactured by Pacific Scientific Corporation. This device measures and reports visible transmittance directly.
"Emissivity" (E) is a measure, or characteristic of both absorption and reflectance of light at given wavelengths. It is usually represented by the formula: EQU E=1-Reflectance.sub.film
For architectural purposes, emissivity values become quite important in the so-called "mid range", sometimes also called the "far range" of the infrared spectrum, i.e. about 2500-40,000 nm. The term "emissivity" as used herein, is thus used to refer to emissivity values measured in this infrared range as specified by the 1991 Proposed ASTM Standard for measuring infrared energy to calculate emittance, as proposed by the Primary Glass Manufacturers' Council and entitled "Test Method for Measuring and Calculating Emittance of Architectural Flat Glass Products Using Radiometric Measurements". This Standard, and its provisions, are incorporated herein by reference. In this Standard, emissivity is broken into two components, hemispherical emissivity (E.sub.h) and normal emissivity (E.sub.n).
The actual accumulation of data for measurement of such emissivity values is conventional and may be done by using, for example, a Beckman Model 4260 spectrophotometer with "VW" attachment (Beckman Scientific Inst. Corp.). This spectrophotometer measures reflectance versus wavelength, and from this, emissivity is calculated using the aforesaid 1991 Proposed ASTM Standard which has been incorporated herein by reference.
Another term employed herein is "sheet resistance". Sheet resistance (R.sub.s) is a well known term in the art and is used herein in accordance with its well known meaning. Generally speaking, this term refers to the resistance in ohms for any square of a layer system on a glass substrate to an electric current passed through the layer system. Sheet resistance is an indication of how well the layer is reflecting infrared energy, and is thus often used along with emissivity as a measure of this characteristic, so important in many architectural glasses. "Sheet resistance" is conveniently measured by using a 4-point probe ohmmeter, such as a dispensable 4-point resistivity probe with a Magnetron Instruments Corp. head, Model M-800 produced by Signatone Corp. of Santa Clara, Calif.
As stated above, for many architectrual purposes it is desirable to have as low an emissivity and R.sub.s value as feasible, such that the glass window is reflecting substantial amounts of the infrared energy impinging on the glass. Generally speaking, "low-E" (i.e. low emissivity) glasses are considered to be those glasses which have a hemispherical emissivity (E.sub.h) of less than about 0.16 and a normal emissivity (E.sub.n) of less than about 0.12. Preferably, E.sub.h is about 0.13 or less, and E.sub.n is about 0.10 or less. At the same time, sheet resistance (R.sub.s) is, therefore, preferably less than about 10.5 ohms/.sub.square. Such glasses, to be commercially acceptable, usually are required to transmit as much visible light as possible, often about 76% or more using the Illuminant C technique for measuring transmittance in glasses of about 2 mm-6 mm thick. Visible transmittance, in this respect, should more preferably be at least about 78% or greater for glasses between about 2 mm-6 mm thick. Even more preferably, visible transmittance should be about 80% or greater, and still most preferably, greater than about 80%.
The technique of creating architectural glass by magnetron sputter-coating multiple layers of metal and/or metal oxides or nitrides onto float glass sheets is well known and a large number of permutations and combinations of known metals (e.g. Ag, Au, etc.), oxides and nitrides have been attempted and reported. Such techniques may employ either planar or tubular targets, or a combination of both, in multi-target zones to achieve their desired results. Exemplary of preferred apparatus for use in this invention, and known in the art, is a magnetron sputter-coater sold by Airco Corporation. This commercially available device is disclosed in U.S. Pat. Nos. 4,356,073 and 4,422,916, respectively. The disclosures of these patents are incorporated herein by reference.
In particular, it has been known to use the aforesaid Airco sputter-coater to produce architectural glasses having a layering system, sequentially from the glass (e.g. standard float glass) outwardly, as follows: EQU Si.sub.3 N.sub.4 /Ni:Cr/Ag/Ni:Cr/Si.sub.3 N.sub.4
in which it has been found in practice that the Ni:Cr alloy is 80/20 by weight Ni/Cr, respectively (i.e. nichrome), and wherein the two nichrome layers are reported as being 7 .ANG. thick, the Ag layer is specified as being only about 70 .ANG. thick [except that it is stated that the silver may be about 100 .ANG. thick], and the Si.sub.3 N.sub.4 layers are relatively thicker (e.g. 320 .ANG. for the undercoat and about 450 .ANG. for the overcoat). In reality, because of its thinness (i.e. about 70 .ANG.), the silver (Ag) layer has been found, in practice, to actually be rather semi-continuous in nature.
FIG. 1 (explained more fully below) schematically illustrates a typical Airco sputter-coater as referenced above, used to produce this known Airco product. With reference to FIG. 1, Zones 1, 2, 4 and 5 are made up of silicon (Si) tubular targets ("t") and sputtering is conducted in a 100% N.sub.2 atmosphere. Zone 3 typically employs planar targets "P" and is used to create the three intermediate layers, i.e. Ni:Cr/Ag/Ni:Cr. A 100% argon atmosphere is employed. It was believed, and heretofore has historically been believed in the sputter-coating art, that N.sub.2 adversely affects silver during sputter-coating, and thus care was used to keep Zone 3 substantially free of N.sub.2.
While this coating achieved good "durability" (i.e. the coating was scratch resistant, wear resistant and chemically stable) and thus achieved an important measure of this characteristic as compared to pyrolytic coatings, its other characteristics, in practice, have been found to fall short of the levels of infrared reflectance and visible transmittance characteristics normally desired for low-E architectural glasses. For example, for glass at about 3 mm thick, visible transmittance (Ill. C) is usually only about 76%, E.sub.h is about 0.20-0.22, and E.sub.n is about 0.14-0.17. Both of these emissivity values are rather high. In addition, sheet resistance (R.sub.s) measures a relatively high 15.8 ohms/.sub.sq. (the more acceptable value being about 10.5 or less). Thus, while durability was significantly improved and while these coatings also proved to be compatible with conventional sealants (thus overcoming this problem in the multi-pane window art which normally required "edge deletion" and is now no longer required), solar management qualities were less than optimal for many modern architectural purposes.
In addition to this Airco layer system, other coatings containing silver and/or Ni:Cr as layers for infrared reflectance and other light management purposes have been reported in the patent and scientific literature. See, for example, the Fabry-Perot filters and other prior art coatings and techniques disclosed in U.S. Pat. Nos. 3,682,528 and 4,799,745 (and the prior art discussed and/or cited therein). See also the dielectric, metal sandwiches created in numerous patents including, for example, U.S. Pat. Nos. 4,179,181; 3,698,946; 3,978,273; 3,901,997; and 3,889,026 just to name a few. While such other coatings have been known or reported, it is believed that prior to our invention, none of these prior art disclosures teach or have achieved the ability to employ the highly productive sputter-coating process and, at the same time, achieve an architectural glass which not only approaches or equals the durability of pyrolytic coatings, but which also achieves excellent solar management qualities as well.
It is to be further stated that while the basic Airco apparatus and basic method of operation have been found to be quite acceptable, its productivity was found lacking. The reason for this lowered productivity is related to the assumption, which we have found to be inapplicable to our invention, that silver had to be isolated from N.sub.2 gas during sputtering.
In view of the above, it is apparent that there exists a need in the art for a sputter-coated layer system which approaches or equals the durability of pyrolytic coatings, but which also achieves optimal solar management characteristics, thereby overcoming the problem normally attendant the pyrolytic method. As used herein, the terms "durable" or "durability" are used in accordance with their well known meanings in the art, and reflect, in this respect, a mechanical and chemical resistance to deterioration approaching or equalling that achieved by the pyrolytic process. It is also apparent from the above that there exists a need in the art for a coating created by magnetron sputter-coating which improves upon the transmittance, emissivity and, preferably, also the sheet resistance of those coatings obtained under the Airco process as above-described, as well as improving upon the productivity of this known process.
It is a purpose of this invention to fulfill the above needs, as well as other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.